The Air You Breathe
by Griever11
Summary: Fill for the prompt 'running out of time' from the Castlefanfics Prompt Challenge on Tumblr. Season 4.
Season 4, sometime after Pandora.

* * *

Slow, she has to slow her breathing down. Beckett's biting down so hard on the rubber mouthpiece attached to her canister of air that her jaw hurts, but she really has to control her breathing. The bile rises in her throat as the severity of her predicament hits her. She's probably going to die. _Again._

Her heart pounds beneath her chest and she struggles to keep herself from panicking. The water's so cold that her fingers are becoming numb and despite the wetsuit she has on, the chill is seeping into her bones.

The abandoned military bunker had flooded the moment her team entered it. While they had expected that to happen, they hadn't foreseen the other security measures the military had put in place. Something they had done triggered a fail-safe, and within seconds, they realized that if they didn't hightail it out of there, they were going to be trapped in one of the many flooding chambers.

In their mad scramble to exit the underwater base, Beckett had gotten disoriented. One minute she's right behind Esposito, and the next, a metal door was slamming down from the ceiling, trapping her left leg against the floor. Unable to call out for help because she needed to keep her mouthpiece in, the rest of her team swam away unaware.

She'd struggled with the heavy door, using everything she had in her to lift it up to free her leg. Being submerged in water was a hindrance, her movements slower and less precise than she would have liked. Her muscles screamed at her, but eventually, she managed to lift the door up an inch and wrench her foot free but not before slipped her flashlight between the heavy metal door and the floor.

She had faced the light outwards so when her team comes back they can find their way to her again. It does means she's now surrounded by watery darkness, backed into the corner of the room struggling to regulate her breathing.

More than a few minutes have gone by since then and with every passing second she loses a tiny bit of hope. She can't afford to take deep breaths anymore and the realization fills her with dread. The light on the electronic display on her canister of air flashes in the darkness, warning her of the declining O2 levels.

The mask she's wearing has fogged up, clouding her vision but she doesn't want to remove it. Can't be bothered to - it's not like she can see anything in the dark anyway.

Water swirls and churns around her and enveloped in deathly silence, eerie and haunting. Her weighted vest anchors her to the floor and she has half a mind to remove it but there's no point in doing that. She's trapped in the room and all that will do is make her float to the ceiling.

The pain in her foot has lessened to a dull throb - something she's grateful for. She doesn't think she'd be able to deal with both the discomfort as well as trying not to suffocate to death. She experiments with it, hoping that maybe if she can move it, she might be able to have more of a leverage, a better angle to lift the door, but-

No.

The excruciating pain shoots up her leg, sending shockwaves pulsing through her entire body and she hisses out loud. She's wasting precious air, but god, it's unbearable and she gulps down a few more mouthfuls of air before slumping back against the wall.

She doesn't fight back the tears - doesn't have the strength to anymore, and her tears collect at the bottom of her face mask as she sucks the last bits of air into her lungs.

She lets her eyelids drift shut, chest heaving as she gives in to the inevitable. She's dying. Might as well accept it with dignity.

Kate Beckett's actually dying.

With only a few bars of air left, she opens her eyes one last time so she can take it all in, to have one last look at the blur of a room she'll call her final resting place. Though, instead of the stark darkness that she's expecting, a ghostly image swims before her.

She chalks it up to hallucinations, the lack of oxygen getting to her brain - because the man in front of her is _him_. Castle. Castle and his bright blue eyes peering at her through the darkness. Of _course_ it would be him that her brain conjures up, she thinks. Even in death, he can't leave her alone. Always butting into her business. Stupid man. She blinks at the mirage but he doesn't disappear so she allows herself to smile.

It's fine. There's no one else she'd want to see before she dies anyway. She's come to accept the fact that no matter what, he'll always be a part of her. Even when she'd been lying in a pool of her own blood, toeing the line between life and death, pain radiating from her chest, his face had been the last thing she saw before she gave in.

Castle. It's always been Castle. Why would it be any different now?

She can see raw pain and anguish reflected in Ghost Castle's eyes and she wants to get rid of it. I'm okay, she wants to tell him. It's okay, Castle. She tries to reach for him but her fingers close in on nothing but water. Disappointment descends upon her but before she succumbs to the darkness, she swears, through the murky depths of her mind, that she hears him telling her she's okay too.

* * *

 **ONE HOUR AGO**

The platform where the team is getting prepped has descended into chaos. People are running to and fro, yelling to one another, tripping over equipment strewn all around the floor. Castle can only look on helplessly, barred from entering the prep zone, palms sweating, swallowing down the dread creeping in his veins.

The marines had been adamant that he stay out of their way. Under no circumstances were they allowing a civilian to participate in their operation, so he's being sidelined much to his dismay. The rest of his team - Ryan, Esposito, and Beckett - are being fitted with the gear that will take them down into the dark depths of the river; weighted vests and handheld canisters of air for their dive down.

Had this been any other time, in any other situation, he'd have been ecstatic over the idea that the military had built a secret underwater base in one of the deepest parts of the Hudson river. But not today. Not when they're sending his team, his friends, _Beckett_ \- down to retrieve a body that may or may not be stashed away in the underwater bunker. Sure, they'll be accompanied by a well-trained marine, but that fact doesn't reassure him.

 _He_ won't have her back. That's the problem.

Castle clenches and unclenches his fists as he watches Ryan and Esposito conduct buddy checks on one another. It's almost time for them to go down and he scans the crowd looking for Beckett. She's nowhere to be seen and invisible fingers curl around his heart, squeezing hard as panic threatens to overcome him.

"Hey, hey, I'm here."

Beckett materializes in front of him seemingly out of thin air and he lets out a sigh of relief. Her vest is strapped tight over her wetsuit, and he's so worried he doesn't even dwell on the fact that she's _in a wetsuit_.

He tries to relax, but the tension still reverberates through him. He's wary of the two burly men standing by his side, ready to stop him from doing anything stupid but he risks it. He's allowed one small step towards her before the two men clear their throats and shake their heads.

"Let me come with you," he pleads. He can't explain it, can't form the words to describe the deep despair that tingles in his bones. The churning in his gut tells him that it's a bad idea, but all he can do is swallow and lick his lips. "I'm your _partner_."

Her eyes stare into his, eyebrows furrowed. If she thinks his behavior is unusual she doesn't verbalize it. She reaches a hand out to him, curling her fingers around his bicep. "I'll be fine. We'll be down there half an hour, tops."

Her words do nothing to allay his fear and he needs more. "Tell me again," he insists. "From the beginning."

Nodding, Beckett repeats the game plan to him. "We dive down about a few feet deep to get to the access tunnel. At the end of the tunnel, there's a blast door that we enter to get to the main room. Corporal Baker doesn't know what the conditions are like in there so we'll take a look around, find our body and come back up. Simple, see?"

Castle's still not convinced, not when every fiber of his being is screaming that all this is a bad idea. "Still don't like it," he mutters.

"Think about it this way, when I get out we can compare and contrast this experience with the last time we were hanging out in the Hudson."

Her attempt at humor falls flat, but he appreciates the effort. He tries, for her. He'll always try, and despite the complete desolation he's feeling, he manages to force a smile onto his face. "Beckett, I-"

"Detective, you ready?"

A gruff voice interrupts him and he feels like punching the guy in the face. For once, just _once_ , can't he have e a single uninterrupted moment with her? He grits his teeth and peers over Beckett's shoulder at the man the voice belongs to. Ryan and Esposito flank him, wearing matching apologetic looks on their faces as they nod at him.

It's time for them to go.

His eyes return to Beckett and she squeezes his arm one last time in reassurance. "I'll be okay. And when we come back, we can-"

"Detective, we need to go _now_!"

Shrugging apologetically at him, Beckett glances back at her team and scurries off. They perform one last quick check of their gear and pull on their masks before diving off the edge, disappearing into the crashing waves.

* * *

 **HALF AN HOUR LATER**

He's panicking. Full blown, heart racing a million miles an hour, sweating buckets, panicking. He knew it. He felt it in his bones that something was going to go wrong and look where they are now. They've lost all contact with the Beckett and the boys, their GPS locators going on the fritz the moment they entered the base. They've had no correspondence since then and it fills him with gut-wrenching despair.

They only had about forty minutes worth of air in their tanks. They'd reassured him that it would be more than enough - that everything was going to work out but now their time's nearly up. Fear spreads through him at the thought that they could possibly-

"Incoming!"

He whips his head around so fast he might have a sore neck the next day, but he doesn't care. Someone's half bent over the edge of the boat, hauling someone else up and his heart soars. Soon enough more people are hurrying to the edge and Castle follows suit, barging past the men to see for himself that Beckett's okay.

"They're okay!" someone shouts. "Our men are okay!"

Our men.

 _Men._

No.

Ryan, Esposito, and the man who was supposed to Beckett's buddy emerge from the crowd, shivering from the cold, dripping wet.

"Where's Beckett?!" Castle bellows. The three men in front of him flinch at the thunder of his voice but he ignores them.

"Castle, she was right behind us, I swear-"

"-she was with us, I promise, bro. She can't be far behind."

"The place was booby trapped and flooding, we had to get out of there before we ran out of air."

He hears them, but he's not listening. His entire body trembles with rage, a million thoughts swirling in his head. They _left_ her. They left her down there, in a flooding bunker with no air-

" _You left her to die_?" Barely above a whisper, his voice still manages to convey the fury he's barely containing. "You're her partners, you're supposed to have her back!"

He barrels past the other people around him, forcing his way to the equipment pile in the corner of the boat. He takes no heed of anything around him, shrugs off the hands trying to hold him back.

There's only one canister of air left in the pile and he makes a grab for it, wrestling with the arms closing in around him. He doesn't bother looking for a mask. He doesn't need one. He's fine. He has air and that's all he needs. He inserts the rubber mouthpiece between his lips and bites down. He sucks in a small experimental breath and when he's sure it's working, he whirls around to make his way back to the side of the boat.

He's not sure how he manages to pull it off, but he does. With strength he doesn't realize he possesses, he muscles his way past the shouting men, pushes Ryan and Esposito away and leaps over the side of the boat.

* * *

He doesn't know how long he's been swimming but finds the access tunnel soon after leaving the boat. The swim down had been difficult; he's an experienced scuba diver but without the weights to aid him, it had taken him far more effort than he expected. Eventually, he makes it to the entrance of the access tunnel, a nondescript circular manhole along the side of a solid rock wall.

He pries it open with ease, hope roaring in his veins, determination dictating his every stroke in the water. He makes his way down the narrow tunnel, feeling his way in the dark, swimming aimlessly until he spots the glimmer of light in his peripheral vision. His eyes are stinging from the water but it's there, flickering in the distance. Every one of his limbs are numb from the cold but be perseveres on with renewed vigor.

The source of light turns out to be from a flashlight, jammed between a solid metal door and the floor. The yellow shimmer of the bulb cuts through the darkness of the water like hope lighting up the bleakness in his heart.

She's here.

He can _feel_ it.

Making sure the canister of air is secure between his lips, he lowers himself to the ground. He's floating at a weird angle, but he makes do. The tips of his shoes only just fit through the gap and he slides them in, anchoring himself to the ground. He crouches down, steadies himself and heaves.

It doesn't budge.

Castle's stomach sinks like lead, the faint glimmer of hope extinguishing just as quickly as it appeared. It's too heavy and he doesn't have enough leverage to lift it up properly. He tries again, not willing to give up. His forearms scream at the exertion, his back straining with the effort and even in the water he can feel his knees shaking with the effort.

He's so close. Beckett _has_ to be behind the door and he focuses on that, concentrating on the fact that all he needs to do is lift, and she'll be right there. His shoulders ache but he pushes on.

He just needs to get to her, he repeats to himself. That's all he needs to do. Then at least ..

Then at least if they both die down here, they won't die alone.

The thought gives him a new burst of energy. Bearing down, he digs his heels into the ground, arches his back and with his thighs quivering, tries to raise it one more time. The door finally moves, groaning as it slides up, though the sound is muted underwater. He puts his shoulders into it, pushes a little bit more and soon enough he's made enough space for him to crawl through.

His broad shoulders scrape against the door, probably drawing blood but he's so numb he feels none of the pain. The chamber is dark and for a millisecond he entertains the thought that maybe it had all been for nothing and she's not even here-

Until he sees her.

Crumpled in a heap, motionless, lifeless. Her canister of air still hangs from her lips, it's warning light flashing, but she's so still, he's not sure if...

He doesn't want to think about being too late. Gritting his teeth, he pushes the morbid thoughts out of his head and rushes to her, his momentum causing him to almost collide with her limp form. He won't give up on her.

Beckett's lips are blue - too blue. Her mask obstructs most of her face but the skin that he can see is pale, and he'll bet his Ferrari that despite the wetsuit she has on, she's freezing.

His fingers reach around the back of her head, grapples with the elastic band of her mask and he rips it off. It floats away into the dark abyss, and he runs his fingers over her face, willing her to open her eyes - show him some sign she's still alive.

He doesn't realize he's crying, but he's gasping for air with every anguished breath and it's bad - he needs to conserve it so he forces himself to stop. He hauls her to him, arms closing in on her, banding around her back as he cuddles her into his body. Her vest is weighted enough to keep them from floating up so he holds on, wills what's left of his body heat to warm her up.

And then her eyes open.

They're open, blinking, and relief travels through his entire body. She's alive. She's okay. He feels her chest expand as she tries to breathe in non-existent air. She struggles in his arms, panic in her eyes as she chokes, gurgling in the water. Her empty can of air floats away and a new terror sinks in.

He turns around and when he sees the now solidly shut door, it confirms his fears.

They won't escape.

He faces Beckett again and deep in his heart he knows what he's going to do. She stops struggling and merely peers at him as if she's read his mind.

Castle releases his hold on her for a second and she reaches her hand out to him as if she's trying to keep him with her. Her fist closes around nothing - he's floating a little too far away without her anchoring him down. She needn't have worried anyway. Nothing in this world would have kept him from her - especially not now.

He just needs...

He needs one last look at her. Needs to memorize her face, wants to etch it into his memories. He's already told her once that he loved her and it seems inconsequential now to repeat it. He doesn't care that she's never said it back - all he's ever wanted was to love her and he's had that. He's loved her for so long, and it's enough.

He breathes in one last deep breath and pulls the can of air out of his mouth. Swimming back to her, he notices she's shut her eyes again and he scrambles to fit the mouthpiece between her lips.

"Breathe," he whispers. She won't hear him, not underwater, but he speaks anyway. "Not gonna let you die, Beckett."

With his remaining ounce of strength, he hugs her to him, chest to chest. "They'll come for you Kate, just breathe."

He battles the instinct to open his mouth - he's done his research, he's only going to die faster that way. His chest constricts and he knows he's suffocating. He doesn't have much time left.

Darkness creeps in around the edges of his vision, and he takes one last long look at her and happiness envelops his heart Her eyes are open again, and she's releasing air bubbles as she starts taking in regular breaths.

She's okay.

She's alive, and her team will find her.

He thinks he hears her screaming at him - but it's probably just his imagination. He imagines she's telling him to hold on and that she loves him but he just stares back up at her.

He smiles because he doesn't want her to remember him without one on his face. He fights through the spots of darkness blooming through his vision and the corners of his mouth curl up, grinning at her one last time.

"I love you too," he mouths.

The last thing he sees before he succumbs to the darkness is her eyes - as beautiful and green as the first day they met.

* * *

 _Many thanks for the beta, kangaroo princess!_

Twitter: estheryam  
Tumblr: Griever11


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